UPPER AND LOWER CRITICAL-POINTS - THERMODYNAMIC CONSTRAINTS ON THE SOLUTION PROPERTIES OF SILICATE MELTS

Thermodynamic stability analysis both classical and nonclassical is ap plied to silicate melts to examine the possible occurrence of two liqu id fields with upper and/or lower critical temperatures. The condition s for the existence of an upper critical point in a binary silicate me lt are that the second derivatives of the enthalpy and entropy with re spect to composition are both negative in supercritical melts. The nec essary condition for the existence of a lower critical point is that t he excess entropy is negative in near critical melts. The excess entro py of olivine-rich melts is likely to be negative, implying that a two -liquid field with a lower critical point may be encountered in superl iquidus peridotite melts.